Write pass error detection

ABSTRACT

A data reader is arranged to read data comprising user data  30  and non-user data  32, 34  written across at least two channels of a data-holding medium  10 , said data being arranged into a plurality of data items  26  each containing user data and non-user data, with said non-user data holding information relating to said user data, including write pass number information, and data items written across the said channels at the same time being identified as a set of data items, said data reader holding a current write pass number and having a read head  12  for reading a respective said channel of said data-holding medium  10  to generate a data signal comprising said data items, and processing circuitry  258, 280  arranged to receive and process said data signals of a set of data items, including processing said write pass number information of each of said data items in said set, and causing updating of said current write pass number held by said data reader on the basis of the write pass number information of said data items in said set.

FIELD OF THE INVENTION

This invention provides an improved data storage device, which may be atape drive arranged to receive data from a computer, or the like. Theinvention also provides related methods.

BACKGROUND OF THE INVENTION

An example of a data storage device is the tape drive, which receivesuser data from computers, particularly, but not exclusively to back-upthe user data held on the computer onto a data-holding medium. In suchback-up applications it is of prime importance that the user data isretrievable, since generally, this copy is the back-up copy that willonly be required if the original has been lost or damaged. Therefore,there is an ongoing need to ensure that back-up data storage devices areas robust and secure as possible.

Once user data has been stored on the data-holding medium it can be heldthere for long periods. To recover the user data from the data-holdingmedium the data storage device must read the data-holding medium andregenerate the user data originally stored there. In some devices theuser data backed-up on the data-holding medium accounts for only about40% of the overall information held on the data-holding medium. Theremaining 60% of the information is non-user data, such as headers orerror detection and correction information that attempts to make theuser data as secure as possible.

Therefore, in order to read the user data the storage device mustaccurately detect which is the user data within all of the informationheld on the data-holding medium. In view of the amount of informationother than user data that is held on the data-holding medium, this canbe problematic.

The storage device must also be able to detect and correct as many aspossible of the errors which may have occurred in writing the user datato the data-holding medium or reading the user data from it, using theerror detection and correction information.

The user data is normally split into discrete items, each item includingthe user data, and non-user data including the error detection andcorrection information and a header denoting its position in the writingsequence, a write pass number and header error detection information.

The write pass number indicates how many times the data-holding mediumhas been written to, that is, if it is the fifth time, the write passnumber is 5. When writing, the physical separation between thedata-holding medium and a write head can vary because, for example, ofdirt on the data-holding medium. If the separation is too great, thesignal strength is reduced. Data previously written to the data-holdingmedium is not then erased, and remains on the data-holding medium. Suchdata is known as a drop-in, and can cause errors when reading the datafrom the data-holding medium. However, drop-ins can be detected by thewrite pass number, which is incremented by the data-storage device eachtime the data-holding medium has been written. A drop-in has a lowerwrite pass number than that of the newer signal which surrounds it, andso can be ignored when reading the data, in order to reduce errors.

When reading, the data storage device holds the write pass number,updating the value as it increases. If data has been written to morethan one channel of the data-holding medium, the write pass numberchanges across all the channels at the same time. It is known in thesecircumstances for a data storage device to check the write pass numberfor each data item separately, so that drop-ins can be discarded, andthe write pass value updated as it increases. However, this can lead tothe write pass value being updated wrongly if a write pass number is inerror. Further, if the write pass has in fact changed for all channels,but the first channels read are drop-ins, they may not be detected assuch because they do not cause an update of the value.

SUMMARY OF THE INVENTION

It is an object of the present invention to use the write pass number todetect and discard data items which are in error, particularly where thedata storage device writes a set of several data items at the same timeon different channels.

According to a first aspect of the invention, a data reader is arrangedto read data comprising user data and non-user data written across atleast two channels of a data-holding medium, said data being arrangedinto a plurality of data items each containing user data and non-userdata, with said non-user data holding information relating to said userdata, including write pass number information, and data items writtenacross the said channels at the same time being identified as a set ofdata items, said data reader holding a current write pass number andhaving a read head for reading a respective said channel of saiddata-holding medium to generate a data signal comprising said dataitems, and processing circuitry arranged to receive and process saiddata signals of a set of data items, including processing said writepass number information of each of said data items in said set, andcausing updating of the current write pass number held by said datareader on the basis of the write pass number information of said dataitems in said set.

Thus, the data reader updates the current write pass number on the basisof information from a set of data items written at the same time, ratherthan a single data item, making it less likely to update in error.Further, because the write pass number is updated on the basis of theset of data items, all drop-ins in that set can be discarded followingthe updating.

Preferably, the write pass number information is contained within aheader for each data item, the header including header error detectioninformation, and the processing circuitry processes said header errordetection information and said write pass number information for eachdata item in the set before updating the current write pass number. Inthis way, the processing circuitry can determine which headers of dataitems may contain errors, thus determining whether write passinformation for any of the data items may be in error, and so can beignored. This improves the reliability of the updating.

The processing circuitry preferably causes updating of the current writepass number held by the data reader on the basis of the write passnumber information of the headers of each data item in the set which arecorrect.

The circuitry may update the current write pass number if at least agiven number of the correct headers agree on it. The number may bechosen to perform more or less rigorous checking. Alternatively, thecircuitry may update the current write pass number if there is at leasta given number of correct headers and all of those have the same writepass number. (Again, the number can be chosen as required). Other waysof determining updating of the current write pass number may also beused.

Conveniently, the processing circuitry interrupts processing when thewrite pass number information is such as to cause an update, to enablefurther processing circuitry to check the write pass number informationto confirm the updating.

In a preferred embodiment the data reader has eight read heads, readingeight data channels, although it may have any number of read heads, fromtwo upwards, for example up to sixteen, although any number is possible.The processing circuitry may update the write pass number if say thedata items from six channels have the same write pass number.

According to a second aspect of the invention, we provide a data storagedevice incorporating a data reader according to the first aspect of theinvention.

In the preferred embodiment, the data storage device is a tape drive.Such a tape drive may be arranged to read data held in any of thefollowing formats: LTO (Linear Tape Open), DAT (Digital Audio Tape), DLT(Digital Linear Tape), DDS (Digital Data Storage), or any other format,although in the preferred embodiment the tape is LTO format.

Alternatively, the data storage device may be any one of the following:CDROM drive, DVD ROM/RAM drive, magneto optical storage device, harddrive, floppy drive, or any other form of storage device suitable forstoring digital data.

According to a third aspect of the invention, we provide a method ofreading data comprising user data and non-user data written across atleast two channels of a data-holding medium, said data being arrangedinto a plurality of data items each containing user data and non-userdata, with said non-user data holding information relating to said userdata, including write pass number information and data items writtenacross the said channels at the same time being identified as a set ofdata items, said method comprising:

reading each said channel of said data-holding medium;

generating a data signal comprising said data items for each saidchannel;

processing said data signals of a set of data items includingdetermining a write pass number from said write pass number informationof each of said data items in said set; and

updating a current write pass number held by a data reader on the basisof said write pass numbers of said data items in said set.

Thus, the current write pass number is updated on the basis of the writepass number information from a set of data items written at the sametime rather than a single data item, making it less likely to update inerror. Further, because the write pass number is updated on the basis ofthe set of data items, all drop-ins in that set can be discardedfollowing the updating.

Preferably, where the write pass number information is contained withina header for each data item, with the header including header errordetection information, the step of processing the data signals includesprocessing the header error detection information to determine which ofthe headers is in error. In this way any headers which are in error (andfor which the write pass number may therefore be incorrect) can beignored in considering the write pass number.

The step of processing the data signals then comprises determining whichof the headers of the data items in the set are correct, determining thewrite pass numbers of those data items and comparing the write passnumbers of the data items with the current write pass number held by thedata reader.

The step of comparing the write pass numbers of the data items mayinclude determining how many of the correct headers have the same writepass number, and whether that number of headers exceeds a given number.If so, the write pass number of the data items is compared with thecurrent write pass number to determine whether updating is required.

Alternatively, it may include determining whether there are at least agiven number of correct headers, and whether all of those have the samewrite pass number. If so, the write pass number of the data items iscompared with the current write pass number to determine whetherupdating is required. In either case, the given number may be chosen toperform more or less rigorous checking.

Other methods of comparing the write pass number of the data items maybe used.

According to a fourth aspect of the invention there is provided acomputer readable medium having stored therein instructions for causinga processing unit to execute the method of the third aspect of theinvention.

The computer readable medium, although not limited to, may be any one ofthe following: a floppy disk, a CDROM, a DVD ROM/RAM, a ZIP™ disk, amagneto optical disc, a hard drive, a transmitted signal (including aninternet download, file transfer, etc.).

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention is described by way of example only inthe accompanying drawings, in which:

FIG. 1 is a schematic diagram of a computer connected to a tape driveaccording to the present invention;

FIG. 2 is a schematic diagram showing the main components of the tapedrive of FIG. 1;

FIG. 3 shows the structure into which data received by the tape drive isarranged;

FIG. 4 shows further detail of the data structure of FIG. 3 and how thedata is written to the tape;

FIG. 5 shows further detail of the data structure of FIGS. 3 and 4, andshows the physical arrangement of the data on the tape;

FIG. 6 is a schematic diagram of a formatter for the data;

FIG. 7 shows more detail of data as written to tape;

FIG. 8 shows further detail of data as written to tape;

FIG. 9 shows schematically the position of a read head in relation to atape;

FIGS. 10 a and b show schematically problems that may occur with asignal being read from a tape;

FIG. 11 shows schematically the operation of a hardware block forchecking write pass; and

FIG. 12 is an expansion of part of the block of FIG. 11.

Turning to FIG. 1, a tape drive 2 is shown connected to a computingdevice 4. The computing device 4 may be any device capable of outputtingdata in the correct format to the tape drive 2, but would typically be adevice such as a computer referred to as a PC, an APPLE MAC™, etc. Thesemachines may run a variety of operating systems such as for exampleMICROSOFT WINDOWS™, UNIX, LINUX, MAC OS™, BEOS™. Generally, because ofthe high cost of the tape drive 2 it would be connected to a high valuecomputer such as a network server running WINDOWS NT™ or UNIX.

A connection 6, in this case a SCSI link, is provided between thecomputing device 4 and the tape drive 2, which allows data to betransferred between the two devices. The tape drive 2 contains controlcircuitry 8, which includes a buffer capable of receiving and bufferingdata received from the computing device 2. A tape 10 has been insertedinto the tape drive and is capable of having data written thereto andread therefrom by a set of write and read heads 12. In this embodimentthere are eight read and eight write heads. The tape drive correspondsto the LTO format and typically receives tapes having a capacity of theorder of 100 Gbytes.

The processing circuitry further comprises memory in which data readfrom the tape is stored whilst it is being decoded, together withelectronics that is arranged to read and decode data from the tape 10.

Data sent by such computing devices is generally sent in bursts, whichresults in packets of data 13 that need to be smoothed in order thatthey can be sequentially recorded by the tape drive. Therefore, thebuffer within the control circuitry 8 buffers these bursts and allowsdata to be continuously 14 written to the tape 10.

The control circuitry is shown in more detail in FIG. 2, which shows anumber of portions of the control circuitry 8. The computing device isrepresented by the left most box of the Figure. The control circuitry 8comprises a burst buffer 16 that has a capacity of 128 Kbytes and isarranged to receive data from the computing device 4. A logicalformatter 18 is provided to perform initial processing of the datareceived by the burst buffer 16. A main buffer 20 is provided having acapacity of 16 Mbytes and is arranged to hold data that is waiting to bewritten to the tape 10, and also holds data that is being read from thetape 10 before being sent to the computing device 4. The final blockshown in FIG. 2 is the physical formatting block 22, which performsfurther processing on the data before it can be written to the tape 10,details of which will be given below.

Data received by the tape drive 2 from the computing device 4 is firstpassed to the burst buffer 16. The burst buffer 16 is required to ensurethat the tape drive 2 can receive the high speed bursts of data sent bythe computing device 4, which may otherwise be received too rapidly forthe logical formatter 18 to process in time. The burst buffer 16 is of aFirst In First Out (FIFO) nature so that the order of the data ismaintained as it is passed to the logical formatter 18.

The logical formatter 18 compresses the data received and arranges itinto a first data structure described hereinafter. Once the data hasbeen processed in this manner it is passed to the main buffer 20, alsoof a FIFO nature, to await further processing before being written tothe tape 10. The capacity of the main buffer 20 is much greater thanthat of the burst buffer 16 so that it can act as a reservoir ofinformation should data be received from the computing device 4 at toogreat a rate, and can be used to allow writing to continue should datatransmission from the computing device 4 be suspended.

The physical formatter 22 handles the writing of the data to the tape,which includes read while writing retries (RWW retries), generation offirst and second levels of error correction (C1 and C2), generation ofheaders, RLL modulation, sync. fields, and provides data recoveryalgorithms. These terms will be expanded upon hereinafter.

As written to the tape 10, the data is arranged in a data structure 24,or dataset, as shown in FIG. 3, details of which are as follows. Thedataset typically holds 400 Kbytes of compressed data, and comprises amatrix of 64×16 C1 codeword pairs (CCP) 26 and there are therefore 1024CCPs within a dataset. Each column of the matrix is referred to as asub-dataset 28, and there are thus 16 sub-datasets within a dataset.

Each CCP, as its name suggests, comprises two code words, eachcontaining 234 bytes of user data, together with 6 bytes of parityinformation (C1 error correction data), which allows the detection andcorrection of 3 bytes in error within any CCP. Therefore, each CCPcomprises 468 bytes of user data 30 and 12 bytes of parity information32. The CCP is also headed by a 10 byte header 34.

Rows zero to fifty-three 36 of the dataset 24 hold user data and C1parity information. Rows fifty-four to sixty-three hold data providingthe second level of error correction, C2 parity information.

In general, when the physical formatter 22 writes data to the tape 10 itwrites the datasets 24 sequentially, each as a codeword quad set (CQset) 38, as shown in FIG. 4. This shows that row zero is written first,then row one, up to row 63. Each row is written across all the writeheads 12 (channel 0 to channel 7). Each CQ set 38 can be represented asa 2×8 matrix, with each cell of the matrix containing a CCP 26 from thedataset. Each row of the 2×8 matrix is written by a separate write head12, thus splitting the CQ set 38 across the tape 10.

Thus, the 1024 CCPs 26 from a dataset 24 are written as 64 CQ sets, asshown in FIG. 5. Between each dataset, a dataset separator (DSS) isrecorded on the tape 10.

The operation of the physical formatter 22 is shown in more detail inFIG. 6. The physical formatter 22 comprises the buffer 20, a writecontroller 222 controlling a write chain controller 224, and a readcontroller 226 controlling a read chain controller 228. The write chaincontroller and the read chain controller both interact with a functionprocessing block 230, which generates the C1 and C2 parity bytes, sendsdata to a CCQ writer 234 for writing onto the tape channels, andreceives data read from the tape channels by a CCQ reader 236. Thephysical formatter 22 is executed as hardware, with the exception of thewrite controller 222 and the read controller 226, which are firmware.

The write chain controller 224 operates the function block 230 togenerate a CCP 26 from the data in the buffer 20, complete write C1 andC2 error correction information. The write chain controller 224 alsogenerates the 10 header bytes 34, which are added by the function block230.

The CCP 26 is then passed from the function block 230 to the CCQ writer234, along with further information from the write chain controller 224,including whether it is the first or the second in a CQ set 38, andwhether it should be preceded by a dataset separator DSS, and whichchannel (0 to 7) it should be written to.

The information in the header 34 is critical, and includes a designatorof its position in the dataset matrix 24 (a number from 0 to 1023), adataset number, a write pass number (to be explained in more detailbelow), an absolute CQ sequence number (all generated by the write chaincontroller 224), and two Reed Solomon header parity bytes, which aregenerated by the function block 230. These header parity bytes enableerrors in the header 34 to be detected, but not necessarily corrected.

The CCPs 26 passed to the CCQ writer 234 are allocated to a particularchannel (0 to 7). Further processing adds synchronisation (sync) fieldsbefore each header 34 (see FIG. 7). This enables headers 34 to berecognised more easily when the data is read.

As shown in FIG. 8 three separate sync fields are used: a forward sync46, a resync 48 and a back sync 50. The forward sync 46 is positionedbefore the header 34 of the first CCP 26 of a CQ set 38. The resync 48is positioned between the two CCPs 26 of a CQ set 38 (i.e. after theparity data 32 of the first CCP 26 and before the header 33 of thesecond CCP 26). The back sync 50 is positioned after the parity data 32of the second codeword pair 26 within the CQ set 38.

The forward sync 46 is preceded by a VFO field 52 which comprises thedata 000010 followed by a number of occurrences of the bit sequence101010. The back sync field 50 is followed by a VFO field 53 thatcomprises the data 000010 followed by a number of occurrences of the bitsequence 101010. The VFO field 52 is easily detectable by the processingcircuitry reading data from the tape 10, and alerts it to the fact aforward sync field 46 is to follow. The back sync 50 and VFO 53 are usedin a similar way when the tape 10 is read backwards. The portion of thetape comprising a forward sync 46 to a back sync 50 comprises asynchronised CQ set 38. The headers 33, 34 contain information as to theidentity of the data and the reading of the headers determines how theprocessing circuitry decodes the data. A DSS is put at the beginning ofa dataset.

The dataset is then written to the tape 10 by the eight write heads 12according to the channels (0 to 7) assigned by the write chaincontroller. When writing, the write pass number contained in the header34 is of importance. As can be seen in FIG. 9, when writing data, thephysical separation X between the write heads 12 and tape 10 can vary.If the write head 12 moved away from the tape 10 when data was beingwritten (i.e. X increased), then when that data is read back the signalstrength at the point corresponding to the increase in X during writingwill be much weaker. This is represented in FIG. 10 a in which thesignal 68 is weakened in the region 70. Such regions are referred to asregions of drop-out. The increased distance X can be caused by a numberof factors, including the presence of dirt on the tape 10 and ripples inthe tape 10.

Whilst the tape 10 contains no information then a drop-out region 70simply results in a loss of signal during reading, and would generate aread while writing retry (as explained below). However, if the tape 10contained information that was being overwritten then because of thereduced field during writing the existing data would not be erased andwould remain on the tape 10 and this is shown in FIG. 10; the new signal68 is shown with a drop-out region 70 as in FIG. 10 a, but an existingsignal 72 remains in this drop-out region. This existing signal isreferred to a region of drop-in.

Drop-in regions must be accounted for during reading of information fromthe tape 10, and the write pass number described above is used toachieve this. All data that is written to the tape 10 is written with awrite pass number, which for a particular tape is incremented each timedata is written thereto. Consequently, a drop-in region of existingsignal 72 will have a lower write pass number than the newer signal 68that surrounds it. If the write pass drops during the middle of adataset as data is being read from the tape 10, this indicates that aregion of drop-in has been encountered. The current write pass number isheld in the CCQ reader 236.

The data being written to the tape 10 is also read by the eight readheads. The data read is passed to the CCQ reader 236, where it isprocessed, as explained below, before being passed to the function block230 for error detection and correction, and for checking by the readchain controller 228. If the tape drive is in Read While Writing mode,the write chain controller 234 checks the CCPs to determine which CQsets 38 are in error, and so need rewriting to the tape 10.

If the tape drive is in Reading mode, that is, for restoration of data,the CCPs 26 are passed to the buffer 20 to await sending back to thecomputer device 4.

The CCQ reader 236 is arranged to detect and in particular to correcterrors in the CCP headers 34 before the CCPs 26 are passed to thefunction block 230. This is advantageous, as it increases the number ofCCPs 26 which can be used to recover data; if the header errors cannotbe corrected the CCP 26 cannot be used and will require the CQ set to berewritten (in RWW mode) or the data to be lost (in restoration mode).The CCQ reader 236 also looks at the write pass number of each CCP 26,enabling drop-ins to be filtered out by the CCQ reader 236. This ensuresthat the CCPs 26 passed to the function block 230 are as error-free aspossible.

In general terms, the CCQ reader 236 gets a data signal from all theread heads, each head passing data through a separate channel (0 to 7).The CCQ reader 236 has a processing block 250 which looks for a VFOsignal 52, followed by a forward sync 46, so that the header of a CCP 26can be detected. Once a CCP 26 has been detected, it is processed in theblock 250, including for each CCP a write pass check, and a headerparity check, to establish any headers 34 that are in error.

The block 250 discards any CCPs 26 that are drop-ins, and corrects theheaders 34 if possible. Then CCPs without header errors are multiplexedto the function block 230 for error correction and further processing bythe read chain controller.

In order to correct errors in the CCP headers 34, the CCQ reader 236must identify CCPs 26 which have been written at the same time, as theheaders 34 will contain similar information, so that information fromthe correct headers can be used to interpolate information into theincorrect headers. Because the write heads 12 may not be preciselyaligned, CCPs written simultaneously will not arrive at the CCQ reader236 on all channels simultaneously. It is then necessary to detect whichwere written at the same time, these being known as a CCP set. Detectionof CCP sets can be done by any suitable method.

In accordance with the invention, the CCQ reader 236 performs checks onthe write pass number in the header 34 of each CCP in the CCP set, usingthe processing block 250. The block 250 compares the write pass numberfor all the CCPs in the set, to enable it to update the write passnumber held by the CCQ reader 236. It also filters out CCPs with a writepass number which is less than the current write pass number held by theCCQ reader 236.

The write pass checking is performed by a write pass checker 280 shownin FIG. 11. The checker 280 is part of the processing block 250. Asshown in FIG. 11, the CCPs are received by initial block 282, which isconfigurable to be in ignore, fixed or update mode. Only update mode isrelevant to the present invention. In ignore mode (the left hand side ofthe Figure) no write pass checking is performed at all, and the CCPs aresimply passed to the function block 230 by the final block 286. In fixedmode (the right hand side of the Figure) the write pass number of theCCPs is checked in a filter block 284, which discards any with a writepass number lower than the current value held by the CCQ reader 236. Theremaining CCPs are passed to the function block 230 by the final block286.

In update mode, the CCPs are first passed to an update write pass block288, shown in more detail in FIG. 12. This shows two different ways ofupdating the write pass number.

In the method shown on the left hand side of the Figure, the first block290 looks at the headers 34 of the CCPs in the set which have correctparity, and finds the largest write pass number of those headers. Thesecond block 292 checks whether this write pass number is larger thanthe current write pass value; if no the write pass block 288 is exited,if yes the third block 294 operates to check whether n channels (0 to 7)have this write pass number, where n is chosen from between 1 and 8. Ifno, the write pass block 288 is exited, but if yes the fourth block 296updates the current write pass value held by the CCQ reader 236, andthen the block 288 is exited.

In the method shown on the right hand side of FIG. 14, the first block298 checks whether there are at least n headers with correct parity,where n is chosen from 1 to 8. If no, the block 288 is exited; if yes,the second block 300 checks whether all these headers have the samewrite pass number. If no, the block 288 is exited; if yes the thirdblock 302 checks whether the write pass number is larger than thecurrent value of the write pass held by the CCQ reader 236. If no, theblock 288 is exited; if yes the fourth block 304 updates the write passnumber in the CCQ reader 236, and then the block 288 is exited.

Thus, the first method takes all the headers 34 of the CCPs in the setwith correct parity, finds the largest write pass number and checks thata given number of headers agree with this before updating.

The second method checks for a given number of headers 34 in the setwith correct parity, and checks that they all agree before updating.This method is more rigorous than the first one.

Further, the choice of n in each method makes the method more or lesslikely to update the write pass. In each method, n is likely to bebetween 2 and 6; a low value of n makes updating more likely, while ahigher value makes updating less likely. The choice of n, which isconfigurable, enables the method to be tailored to suit the particulardata storage device.

It will be appreciated that other methods of deciding on updating couldbe used. However, any method is likely to look at the headers 34 withcorrect parity and how many have the same write pass number. In allcases, it is important to ensure that the write pass is not updatedwrongly, as this will result in more data being discarded as drop-ins.

In order to guard against wrong updating of the current write passnumber in the CCQ reader 236, if the write pass number is changed by theblock 288, the CCQ reader 236 generates an interrupt, enabling firmwareto check the new value and/or to use a more sophisticated write passchecking method if required.

1. A data reader arranged to read user data and non-user data writtenacross at least two channels of a data-holding medium, said data beingarranged into a plurality of data items each including user data andnon-user data, said non-user data holding information relating to saiduser data, the information including write pass number information, anddata items written across the said channels at the same time beingidentified as a set of data items, said write pass number informationbeing included in a header for each said data item, said headerincluding header error detection information, said data readercomprising an arrangement for holding a current write pass number, aread head for reading a respective said channel of said data-holdingmedium and for thereby deriving a data signal having said data items,and processing circuitry arranged to (a) receive and process said datasignals of a set of data items, including processing said write passnumber information of each of said data items in said set, (b) receiveupdating of said current write pass number held by said data reader onthe basis of the write pass number information of said data items insaid set, and (c) process (i) said header error detection informationand (ii) said write pass number information for each said data item insaid set before updating said current write pass number.
 2. A datareader according to claim 1, wherein said processing circuitry isarranged to update said current write pass number held by said datareader on the basis of said write pass number information of saidheaders of each said data item in said set which are correct.
 3. A datareader according to claim 1, wherein said circuitry updates said writepass number if at least a given number of said correct headers agree onit.
 4. A data reader according to claim 3, wherein said given number isvariable.
 5. A data reader according to claim 1, wherein said circuitryupdates said write pass number if there is at least a given number ofsaid correct headers and all of those have the same write pass number.6. A data reader according to claim 1, wherein said processing circuitryis arranged to interrupt processing in response to said write passnumber information being such as to cause an update, to enable furtherprocessing circuitry to check said write pass number information toconfirm the updating.
 7. A data storage device incorporating a datareader according to claim
 1. 8. A method of reading user data andnon-user data written across at least two channels of a data-holdingmedium, said data being arranged into a plurality of data items eachincluding user data and non-user data, said non-user data holdinginformation relating to said user data, including write pass numberinformation, said write pass number information being included in aheader for each said data item, said header including header errordetection information, the data items written across the said channelsat the same time being identified as a set of data items, said methodcomprising: reading each said channel of said data-holding medium;generating a data signal comprising said data items for each saidchannel; processing said data signals of a set of data items, theprocessing including (a) determining a write pass number from said writepass number information of each of said data items in said set, (b)determining which of said headers is in error by processing said headererror detection information; and updating a current write pass numberheld by a data reader on the basis of said write pass numbers of saiddata items in said set.
 9. A method according to claim 8, wherein saidstep of processing said data signals comprises determining which of saidheaders of said data items in said set are correct, determining thewrite pass numbers of those data items and comparing the write passnumbers of said data items with the current write pass number held bysaid data reader.
 10. A method according to claim 9, wherein said stepof comparing the write pass numbers of said data items includesdetermining how many of said correct headers have the same write passnumber, and whether that number of headers exceeds a given number.
 11. Amethod according to claim 10, wherein said write pass number of saiddata items is compared with said current write pass number to determinewhether updating is required.
 12. A method according to claim 10,wherein said given number is variable.
 13. A method according to claim9, wherein said step of comparing the write pass numbers of said dataitems includes determining whether there are at least a given number ofcorrect headers, and whether all of those have the same write passnumber.